The present invention relates to an electron device which functions as a high-output power transistor employed, for example, in base stations for mobile radios.
In the past, electron emissive elements had a structure provided by the hot cathode method (or an electron gun method). The electron emissive element is provided with a cathode formed of a material having a high melting point such as tungsten (W) and an anode spaced opposite to the cathode. The cathode is heated to high temperatures to launch hot electrons from the solid into a vacuum. Also available is a so-called NEA emissive element which the inventors suggest to replace those employing the hot cathode method. The NEA electron emissive element employs a semiconductor material or an insulating material having a negative electron affinity (NEA). Described below is the principle of an electron device that functions as an electron emissive element (hereinafter referred to as the NEA electron device).
FIG. 1 is a perspective view illustrating the structure of a prior-art NEA electron device that employs aluminum nitride (AlN) as an example of a NEA material. As shown in FIG. 1, the NEA electron device includes an electron supplying layer 101 for supplying electrons and an electron transport layer 102 for transporting the electrons supplied from the electron supplying layer 101 toward the solid surface side. The NEA electron device also includes a surface layer 103 formed of a NEA material and a surface electrode 104 used for the application of a voltage to allow electrons to travel from the electron supplying layer 101 to the surface layer 103.
In this example, the electron supplying layer 101 is formed of an n-type GaN (n-GaN), and the electron transport layer 102 for allowing electrons to travel smoothly from the electron supplying layer 101 to the surface layer 103 is formed of non-doped AlxGa1xe2x88x92xN (where x is a variable increasing in general continuously from 0 to 1) having a graded composition with an Al content ratio x varying continuously. The surface layer 103 is formed of AlN which is an intrinsic NEA material, and the surface electrode is formed of a metal such as platinum (Pt).
Now, described below are the electron affinity that is significant to the basic characteristics of this element and the structure of the electron transport layer that is required for smooth transportation of electrons.
1. Electron Affinity
The xe2x80x9celectron affinityxe2x80x9d in a semiconductor material is defined as the energy required to launch an electron present on the conduction band edge into a vacuum and unique to the material. Now, described below is the concept of xe2x80x9cnegative electron affinity (NEA)xe2x80x9d.
FIGS. 2(a) and (b) are energy band diagrams of semiconductor materials having a negative and positive electron affinity, illustrating the respective energy states. As shown in FIG. 2(b), the electron affinity "khgr"=Evacxe2x88x92Ec greater than 0 in a typical semiconductor, where Ef is the Fermi level of the semiconductor, Ec is the energy level of the conduction band edge, Ev is the energy level of the valence band edge, Eg is the bandgap, and Evac is the vacuum level. That is, the semiconductor has a positive electron affinity. In contrast, for some types of semiconductors, "khgr"=Evacxe2x88x92Ec less than 0 as shown in FIG. 2(a). That is, semiconductors such as AlN have a negative electron affinity.
Now, consider a semiconductor having a positive electron affinity as shown in FIG. 2(b). In this case, to launch an electron present on the conduction band edge into a vacuum, the presence of the energy barrier of a magnitude of "khgr" requires to give the amount of energy to the electron. For electron emission, it is therefore necessary in general to give an energy to an electron by heating or to allow an electron to tunnel the energy barrier by application of a high electric field.
On the other hand, consider a semiconductor having a negative electron affinity as shown in FIG. 2(a). In this case, absence of energy barrier allows an electron present on the conduction band edge of the surface to be easily emitted into a vacuum. In other words, no additional energy is required to launch the electron present on the semiconductor surface into a vacuum.
2. Electron Transport Layer
It is conceivably effective in efficient electron emission to employ, as the surface layer of an electron device for emitting the electron, a material having a substantially zero or negative electron affinity like the one mentioned above. However, no electron is present in general on the conduction band of a NEA material in an equilibrium state. Therefore, it is necessary to efficiently supply electrons in some way to the surface layer formed of a material that allows electrons to be emitted easily.
As shown in FIG. 1, the inventors have suggested a structural example. The structure has an intermediate layer (the electron transport layer 102) having gradually decreasing values of electron affinity to effectively supply electrons from the electron supplying layer 101 (a positive electron affinity), having a number of electrons therein, to the surface layer 103 in a NEA state (a negative electron affinity).
FIGS. 3(a) and (b) are energy band diagrams of the structural example of FIG. 1, provided when no voltage is applied between the electron supplying layer 101 and the surface electrode 104 (an equilibrium state) and a forward bias V is applied therebetween. Here, the structure includes the electron supplying layer 101, the electron transport layer 102, the surface layer 103, and the surface electrode 104. As mentioned above, the electron transport layer 102 is selected from materials that gradually decrease in electron affinity "khgr" toward the surface.
In the equilibrium state shown in FIG. 3(a), there exist a number of electrons in the conduction band of the electron supplying layer 101. However, the high energy level of the conduction band edge of the surface layer 103 prevents the electrons from reaching the outermost surface on the other hand, when a forward bias is applied to such a structure (a positive voltage to the surface electrode side), the energy band is bent as shown in FIG. 3(b). As a result, the gradients of the concentration and the potential cause electrons present in the electron supplying layer 101 to travel toward the surface layer 103. In other words, an electron current flows. In addition, the electron transport layer 102 or (AlxGa1xe2x88x92xN) and the surface layer 103 or (AlN) are non-doped. Accordingly, the electrons injected from the electron supplying layer 101 to the electron transport layer 102 and the surface layer 103 can travel without being captured by recombination with holes or the like. Furthermore, the electron transport layer 102 is continuously graded in composition and thereby no energy barrier, which prevents electrons from traveling, is formed on the conduction band edge. Thus, this is advantageous in that electrons are efficiently transported to the surface.
As described above, the compositionally graded AlxGa1xe2x88x92xN layer is employed as the electron transport layer 102. This allows electrons to efficiently travel from the n-GaN layer having a positive electron affinity to the surface layer 103 (AlN layer) having a negative electron affinity. Then, since the surface layer is in a NEA state, the electrons injected to the electron transport layer 102 and the surface layer 103 can pass easily through the surface electrode 104 to be emitted outwardly into a vacuum or the like.
However, such a phenomenon was also observed in the NEA electron device employing the structure shown in FIG. 1 that the application of a predetermined voltage to the surface electrode 104 would not serve to provide the expected amount of electrons.
A diagnosis of the cause of the phenomenon showed that defects such as fine cracks had occurred in the AlxGa1xe2x88x92xN layer that constituted the electron transport layer 102 and the surface layer 103. That is, the composition of the AlxGa1xe2x88x92xN layer is largely varied to provide significant variations in the bandgap of the electron transport layer 102. This has conceivably caused stress to occur due to variations in lattice constant, resulting in fine cracks. The electrons flowing through the defected portions such as cracks are not supplied to the portion of the surface layer being in the NEA state but flow out to the surface electrode 104 as leakage current. Consequently, this provides a less amount of electrons that pass though the surface electrode 104 to be emitted outwardly and whereby such a problem has been presumably raised that the efficiency of electron emission is lowered.
Incidentally, high-output power transistors, employed in base stations for mobile telephones or employed for wireless LANs, for use with high-frequency signals are conventionally composed of MESFETs or bipolar transistors making use of a GaAs substrate. These elements have advantages of having trackability for high-frequency signals provided by high-mobility electrons in the GaAs substrate and a high breakdown voltage provided by GaAs that has a larger bandgap than Si.
However, conventional MESFETs or bipolar transistors have a breakdown voltage that is defined by a depletion layer produced upon application of a voltage between the gate and the drain or between the base and the collector. This prevents the MESFETs or bipolar transistors from providing breakdown voltages that exceed the limit defined by the physical property of the semiconductor material (GaAs). For example, it is difficult to operate the existing power transistor at voltages of 30V or greater. For this reason, it is necessary to increase the amount of current in order to provide high output (high power). However, there is a drawback that an increase in current would cause an increase in power loss in comparison with an increase in voltage.
It is therefore a first object of the present invention to provide an electron device which is provided with means for preventing leakage current caused by defects such as a crack on the electron transport layer or the surface layer and thereby provides a high efficiency of electron emission.
A second object of the present invention is to make use of electrons that can pass through the conduction band not by tunneling but by conduction to utilize the insulating property, which is intrinsically given to insulators, thereby realizing a junction transistor that can function as a high-output power transistor having a high withstand voltage.
An electron device according to the present invention includes an electron supplying layer and an electron transport layer provided on the electron supplying layer and modulated so that an electron affinity is reduced from the electron supplying layer to a surface layer. The electron device also includes a surface layer provided on the electron transport layer and formed of a material having an electron affinity being negative or close to zero, and a surface electrode for applying a voltage to the electron supplying layer to allow electrons to travel from the electron supplying layer to an outermost surface of the surface layer via the electron transport layer. The electron device further includes a filter layer, disposed between the surface layer and the surface electrode, functioning as a barrier for preventing part of electrons from traveling to the surface electrode, and having an electron affinity equal to or larger than that of the surface layer.
For defects such as cracks present in the electron transport layer, this allows the filter layer disposed between the surface layer and the surface electrode to function as a barrier for preventing electrons from traveling which do not reach a NEA state portion in the surface layer, thereby preventing leakage current from flowing into the surface electrode. In addition, since the electron affinity of the filter layer is larger than that of the surface layer, the filter layer will not serve as a barrier for preventing electrons from traveling which have an energy level equal to or greater than that of the conduction band edge of the surface layer. Accordingly, the presence of the filter layer serves to prevent only the leakage current and emit electrons effectively from the surface layer in response to a voltage applied between the surface electrode and the electron supplying layer.
At least part of the electron transport layer has a bandgap that expands continuously in general from the electron supplying layer to the surface layer and whereby electrons travel preferably smoothly through the electron transport layer.
It is preferable that a region containing the electron transport layer and the surface layer is formed of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61) varying so as to increase the ratio of Al toward the outermost surface.
In this case, it is preferable that the electron transport layer has an Al content ratio x which increases continuously in general from 0 to 0.65 or greater from one end adjacent the electron supplying layer to the other end adjacent the surface layer.
In addition, it is preferable that carrier impurities are not doped in the electron transport layer.
The surface layer is formed of AlxGa1xe2x88x92xN (0.65xe2x89xa6xxe2x89xa61) and whereby a negative electron affinity state can be realized easily on the surface thereof. Accordingly, this is preferable in that such an element can be obtained that has a high efficiency of electron emission.
The filter layer is preferably formed of an insulating material having a positive electron affinity. It is also preferable that the filter layer contains at least any one of aluminum oxide (Al2O3), silicon oxide (SiOx), and silicon nitride (SiNx). It is further preferable that the filter layer contains at least any one of aluminum nitride (AlN), a mixed crystal semiconductor of gallium nitridexe2x80x94aluminum nitride (AlxGa1xe2x88x92xN) (0.65xe2x89xa6xxe2x89xa61), and oxides of these materials.
The electron device further includes the collecting electrode, disposed above and spaced from the surface electrode, for accelerating and controlling electrons emitted outwardly from the surface layer. This is preferable in that mechanisms can be integrated for accelerating and collecting a current of electrons emitted from the surface of the electrode layer by the application of a voltage. That is, the integrated structure of the collecting electrode layer for collecting electrons emitted by applying a voltage between the electron supplying layer and the electrode layer makes it possible to fabricate a compact and high-density electron device that can perform signal amplification and switching operation. The element includes the electron supplying layer/electron transport layer/surface layer/electrode layer, which readily emits electrons as described above, and is adapted to accelerate emitted electrons. This provides advantages of being high in breakdown voltage, low in internal loss, and capable of low voltage drive.
A sealing member is further provided which maintains in a reduced pressure state between the electrode layer and the collecting electrode layer. This allows electrons to be accelerated at high speeds in a vacuum and collected by the collecting electrode, thereby providing a high switching function.
An insulating layer may be further provided which is disposed between the electrode layer and the collecting electrode.
Further provided is a buried layer for confining a region of electrons flowing through the electron transport layer into part of a cross section of the electron transport layer. This allows the current to be condensed, thereby making it possible to increase the efficiency of electron emission from the surface layer.
A junction transistor according to the present invention includes an emitter layer for supplying electrons, an electron transfer layer provided on the emitter layer and adapted to allow supplied electrons to travel therethrough, and a control electrode for applying a voltage to control the amount of electron supply from the emitter layer to the electron transfer layer. The junction transistor also includes a collecting electrode for collecting at least part of electrons supplied from the emitter layer, and an insulating layer interposed between the control electrode and the collecting electrode and having an electron affinity equal to or larger than that of an end portion of the electron transfer layer adjacent the control electrode. The junction transistor is adapted that electrons injected from the electron transfer layer to the insulating layer are adapted to conduct through a conduction band of the insulating layer to reach the collecting electrode.
When a voltage is applied between the control electrode and the emitter layer, this allows electrons to pass through the electron transfer layer from the electron supplying layer and to be then injected from the surface of the electron transfer layer. At this time, since the electron affinity of the insulating layer is larger than that of the outermost surface portion of the electron transfer layer, the injected electrons are allowed to conduct through the conduction band of the insulating layer to reach the collecting electrode. In addition, the insulating layer is interposed between the control electrode and the collecting electrode, thereby making it possible to provide a high breakdown voltage between the collecting electrode and the control electrode. Accordingly, such a junction transistor is obtained which can employ a high voltage to function as a high-output power transistor with low power loss.
The electron affinity of the electron transfer layer is adjusted to be made smaller from the emitter layer toward the control electrode, thereby facilitating injection of electrons into the insulating layer.
The electron transfer layer has a bandgap expanding from the emitter layer to the control electrode and the electron affinity is whereby preferably controlled.
The emitter layer and the electron transfer layer contain a layer formed of nitride, thereby making it easier to reduce the electron affinity as small as possible.
The electron transfer layer is formed of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61) varying so as to increase the ratio of Al toward the outermost surface. This allows a negative electron affinity state to be easily realized on the surface and is preferable in that such an element can be obtained which has a high efficiency of electron injection.
The insulating layer preferably contains at least any one of aluminum oxide (Al2O3), silicon oxide (SiOx), and silicon nitride (SiNx). It is also preferable that the insulating layer contains at least any one of AlN, AlxGa1xe2x88x92xN (0.65xe2x89xa6xxe2x89xa61), and oxides of these materials.
It is preferable that the junction transistor further includes a surface layer disposed between the electron transfer layer and the control electrode and formed of a material having an electron affinity being negative or close to zero.
The junction transistor further includes a filter layer, disposed between the electron transfer layer and the control electrode, functioning as a barrier for preventing electrons from traveling to the control electrode, and having an electron affinity equal to or larger than that of the control electrode. This makes it possible to prevent a leakage current from flowing from the electron transfer layer to the control electrode.
The junction transistor further includes a buried layer for confining a region of electrons flowing in the electron transfer layer to part of a cross section of the electron transfer layer. This allows the current to be condensed to whereby increase the efficiency of electron injection.
It is preferable that the control electrode is disposed across an electron current flowing from the emitter layer to the collecting electrode.